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So a 2,000-square-foot crawl space would need a 40 cfm exhaust fan. It's pulling just a little bit of air from the crawl space. The problem is that you don't really know where the makeup air for the crawl space is coming from. Yes, the code specifies a pathway for air to move into the crawl space from the house, but air doesn't always follow those arrows in the diagrams.
What is air made of? Dry air is primarily made up of nitrogen (78.09%) and oxygen (20.95%). The remaining 1% is made up of argon (0.93%), carbon dioxide (0.039% as of 2010) and other trace gases (0.003%). Water vapor (water in its gaseous state) is also present in the atmosphere in varying amounts, by up to 2%. CARBON DIOXIDE: The amount of carbon dioxide, although small, will likely double in the next 100 years as more petroleum and coal is burned to fuel the world's need for energy.